A polycarbonate has excellent characteristics, such as transparency, heat resistance, and mechanical characteristics, and hence has been used in a wide variety of applications including: casings for an OA apparatus and a home electrical appliance; members in electrical and electronic fields; optical materials, such as various optical disc substrates and lenses; carport roof materials; and various construction materials. In addition, the production amount, and number of applications, of the polycarbonate have been increasing. Therefore, a production method providing high productivity has been required in the production of the polycarbonate.
An interfacial polycondensation method or an ester exchange method has been known as a method of producing the polycarbonate. However, the interfacial polycondensation method involving using a halogen-based organic solvent, such as methylene chloride, has been mainstream in terms of a problem concerning quality or the like.
In the method of producing the polycarbonate based on the interfacial polycondensation method, a polycarbonate powder is obtained as described below. An alkali aqueous solution of a dihydric phenol and phosgene are caused to react with each other in the presence of an organic solvent. An organic solvent solution containing a polycarbonate obtained after the reaction is introduced into a kneader or the like, and is powdered by removing the organic solvent. Then, the resultant powder is pulverized as required. The polycarbonate powder thus obtained contains the used organic solvent at a content as large as from 10 mass % to 50 mass %, and hence the remaining organic solvent has been removed by subjecting the powder to a drying treatment.
As the drying method, in, for example, Patent Document 1, there is a disclosure that a drying treatment is performed at a temperature less than the glass transition temperature of an aromatic polycarbonate resin. In Patent Document 1, there is a disclosure that the drying treatment is performed at a temperature less than the glass transition temperature of the polycarbonate resin, but in Examples thereof, an organic solvent is removed by performing the drying treatment at 145° C. in order that a polycarbonate resin having a glass transition temperature of 149.3° C. may be obtained. When the drying is performed at a temperature that differs from the glass transition temperature by as small as 4.3° C. as described above, a polycarbonate powder fuses to a wall surface in a drying machine to reduce the heat transfer performance of the drying machine, and hence its drying performance reduces in some cases. In addition, the polycarbonate that has adhered to the wall surface in the drying machine peels off in some cases. In particular, when the polycarbonate that has adhered at the time of the production of a high-molecular weight polycarbonate peels off during the drying treatment of a low-molecular weight polycarbonate after the switching of a grade, a problem in that a gel occurs in a molded article to deteriorate the quality of the molded article occurs.